Recent attention has been focused on the in vivo roles of sugar chains and/or complex carbohydrates. For example, factors for determining blood types are glycoproteins, and it is glycolipids that are involved in the functions of the nervous system. Thus, enzymes having the ability to synthesize sugar chains constitute an extremely important key to analyzing physiological activities provided by various sugar chains.
For example, N-acetyl-D-galactosamine (hereinafter also referred to as “GalNAc”) is among the components constituting glycosaminoglycans, as well as being a sugar residue found in various sugar chain structures such as glycosphingolipids and mucin-type sugar chains. Thus, an enzyme transferring GalNAc will serve as an extremely important tool in analyzing the roles of sugar chains in various tissues in vivo.
As described above, attention has been focused on the in vivo roles of sugar chains, but it cannot be said that sufficient headway has been made in analyzing in vivo sugar chain synthesis. This is in part because the mechanism of sugar chain synthesis and the in vivo localization of sugar synthesis have not been fully analyzed. In analyzing the mechanism of sugar chain synthesis, it is necessary to analyze glycosylation enzymes (particularly glycosyltransferases) and to analyze what kind of sugar chains are synthesized by means of the enzymes. To this end, there is a strong demand for searching novel glycosyltransferases and analyzing their functions.
There are some reports of glycosyltransferases having the ability to transfer GalNAc (Non-patent Documents 1 to 4). For example, among human GalNAc transferases, enzymes transferring GalNAc with “β1,4 linkage” are known (Non-patent Document 1) and enzymes using “galactose” as their acceptor substrate are known as enzymes transferring GalNAc with β1,3 linkage (Non-patent Document 2) (“β1,3” or “β3” as used herein refers to a glycosidic linkage between an α-hydroxyl group at the 1-position of a sugar residue in an acceptor substrate and a hydroxyl group at the 3-position of a sugar residue to be transferred and linked thereto).
On the other hand, in higher organisms like humans, no enzyme is known to transfer GalNAc with “β1,3 linkage” to “N-acetylglucosamine” (hereinafter also referred to as “GlcNAc”).
Although there is a report showing that the sugar chain structure in which GalNAc and GlcNAc are linked in a β1,3 fashion was confirmed in sugar chains on neutral glycolipids of fly, a kind of arthropod (Non-patent Document 5), it has been believed that such a sugar chain structure is not present in mammals, particularly in humans, to begin with.    Patent Document 1    International Patent Publication No. WO 01/79556    Non-patent Document 1    Cancer Res. 1993 Nov. 15; 53(22):5395-400: Yamashiro S, Ruan S, Furukawa K, Tai T, Lloyd K O, Shiku H, Furukawa K. Genetic and enzymatic basis for the differential expression of GM2 and GD2 gangliosides in human cancer cell lines.    Non-patent Document 2    Biochim Biophys Acta. 1995 Jan. 3; 1254(1):56-65: Taga S, Tetaud C, Mangeney M, Tursz T, Wiels J. Sequential changes in glycolipid expression during human B cell, differentiation: enzymatic bases.    Non-patent Document 3    Proc Natl Acad Sci USA. 1996 Oct. 1; 93(20):10697-702: Haslam D B, Baenziger J U. Related Articles, Links, Expression cloning of Forssman gly colipid synthetase: a novel member of the histo-blood group ABO gene family.    Non-patent Document 4    Biol. Chem. 1997 Sep. 19; 272(38): 23503-14: Wandall H H, Hassan H, Mirgorodskaya E, Kristensen A K, Roepstorff P, Bennett E P, Nielsen P A, Hollingsworth M A, Burchell J, Taylor-Papadimitriou J, Clausen H. Substrate specificities of three members of the human, UDP-N-acetyl-alpha-D-galactosamine: Polypeptide N-acetylgalactosaminyltransferase family, GalNAc-T1, -T2, and -T3.    Non-patent Document 5    J. Biochem. (Tokyo) 1990 June; 107(6); 899-903: Sugita M. Inagaki F, Naito H, Hori T., Studies on glycosphingolipids in larvae of the green-bottle fly, Lucilia caesar: two neutral glycosphingolipids having large straight oligosaccharide chains with eight and nine sugars.